This invention relates to gallium arsenide semiconductor devices and to a method for their fabrication. More particularly, this invention concerns itself with the fabrication of tellurium implanted submicron N-type layers in gallium arsenide semiconductors and to the use of aluminum nitride as an encapsulating medium during the high temperature annealing step used in the fabrication process.
In recent times, gallium arsenide has become a basic semiconductor material for the production of microwave devices. It exhibits excellent semiconductor properties, particularly at higher temperatures and at both higher and lower frequencies than are usable with germanium or silicon. This is attested to by its increased use in the fabrication of rectifiers, transistors, photoconductors, light sources, light emitters, and maser and laser diodes. The majority of these microwave devices are constructed by epitaxial doping techniques since an N-type diffusion technology in gallium arsenide has not been established. However, even though epitaxial methods are successful, it has been difficult to produce uniform dopant layers less than a few tenths of a micron in thickness.
In an attempt to overcome this problem, it has been found that ion implantation is an effective and efficient process for doping gallium arsenide. It provides accurate control for the doping process and lends itself well to mass production.
In addition to the doping of active regions, ion implantation can be utilized to reduce GaAs contact resistance. For most devices it is essential to have the lowest resistance Ohmic contact possible. The generation of excessive Ohmic heating limits the output power of such devices as laser diodes, impatt diodes and Gunn oscillators.
Some early attempts at the N-type doping of GaAs suggested the use of room temperature implantation and an SiO.sub.2 anneal overcoat. However, room temperature implantation in GaAs was found to lead to lower electrical activity than hot substrate implantation. In addition, gallium readily diffuses through SiO.sub.2, and in the case of tellurium implantation, this loss of gallium has been found to result in Ga vacancy-Te complexes after anneal.
In considering the ancillary problem posed through the use of a SiO.sub.2 overcoat, it was suggested that Si.sub.3 N.sub.4 be used since it provides an excellent mask against gallium or arsenic diffusion.
Unfortunately, the ion implantation of N-type layers in GaAs has not been a consistently reproducible process. Electrical activities over a wide range have been observed for identical implant conditions and substrates. This has been attributed largely to the often poor adherence of sputtered Si.sub.3 N.sub.4 during the annealing process. This problem of adherence may be related to the facts that it is difficult to sputter oxygen-free Si.sub.3 N.sub.4 and the thermal mismatch between Si.sub.3 N.sub.4 and GaAs is large. Also, the composition and strain characteristics of sputtered Si.sub.3 N.sub.4 may be quite different than Si.sub.3 N.sub.4 deposited by other techniques.
In an effort to obtain more uniform results, considerable research was conducted on the effect of changing the anneal overcoat. The objective was to find a dielectric layer with improved adherence and masking qualities that would result in consistently high electrical activity. Sputtered AlN was found to be the most effective anneal overcoat for this work since it has an expansion coefficient of 6.1.times.10.sup.-6 /.degree. C which closely matches the GaAs value. In addition, any oxygen incorporated in the AlN film would be in the form of Al.sub.2 O.sub.3, not SiO.sub.2 as in the case of Si.sub.3 N.sub.4. The process provides an effective method for producing tellurium implanted N-type layers in gallium arsenide.